1. Field of the Invention
The present invention relates to a toner carrier such as a developing roller, a developing device and an image forming apparatus which are used in a copy machine, a facsimile, a printer or the like. More specifically, the present invention relates to a toner carrier and a developing device configured to form a toner image on a latent image carrier by conveying toner particles hopping on an outer circumferential surface thereof to a development region facing the latent image carrier and then by developing an electrostatic latent image on the latent image carrier. Moreover, the present invention relates to an image forming apparatus including the developing device.
2. Description of the Related Art
As a conventional developing device provided in an image forming apparatus, there is known a developing device configured to develop an electrostatic latent image by using toner particles hopping on the outer circumferential surface of a toner carrier such as a developing roller (for example, see Japanese Patent Application Publication No. 2003-255692 (called Patent Document 1 below)). The developing roller as the toner carrier in the developing device disclosed in Patent Document 1 includes a cylindrical member and a pair of conductive electrode bars. The cylindrical member is made of an acrylic resin and is provided with first electrodes and second electrodes which are arranged alternately in a circumferential direction and are electrically isolated from each other. The electrode bars are respectively attached to both end portions of the cylindrical member in a longitudinal direction thereof and are each electrically connected to the first electrodes or the second electrodes.
The electrodes are formed in the following manner. Firstly, in a cutting process, grooves each extending in the longitudinal direction of the cylindrical member are formed in the outer circumferential surface of the cylindrical member. After that, a plating layer is formed on the entire outer circumferential surface of the cylindrical member by electroless nickel plating. Then portions of the plating layer on the outer circumferential surface except inside the grooves are removed in another cutting process. Thereby, the electrodes are formed inside the grooves, respectively. The outer circumferential surface of the cylindrical member including surfaces of the electrodes is entirely coated with a silicone resin.
In the developing roller thus manufactured, an AC power source applies an AC voltage to a pair of electrodes, that is, the first electrode and the second electrode. In the developing roller, an alternating electric field is formed between the first electrode and the second electrode, and thereby causes toner particles to be hopping so as to repeatedly reciprocate between the first electrode and the second electrodes. To be more specific, the developing roller causes toner particles placed above the first electrode to float up and land on the second electrode and then to float up again from the second electrode and land on the first electrode. The developing roller is rotated about its axial center and thereby conveys the toner particles to the development region facing the latent image carrier. The developing roller develops an electrostatic latent image on the latent image carrier by causing the hopping toner to be adsorbed by the electrostatic latent image in the development region.
As described above, the developing roller described in Patent Document 1 develops the electrostatic latent image not by using toner particles adhering to the outer circumferential surface of the developing roller or magnetic carriers, but by using toner particles not exerting the adsorptive power due to the hopping. In this way, the developing roller is capable of performing a low voltage development by causing toner particles to be adsorbed by a portion of the outer circumferential surface of the latent image carrier holding an electrostatic latent image whose potential difference from a non-image portion is only several tens volts.
The developing roller described in Patent Document 1, however, needs the cutting processes for forming the foregoing grooves and for removing the unnecessary portions of the plating layer, and thereby tends to raise a cost with an increase in time required for the processes. In addition, chips generated in the cutting processes may short-circuit the electrodes, or the developing roller may fail to achieve desired accuracy due to the cylindrical member deformed in the cutting process for forming the grooves.
To solve the problems of the developing roller described in Patent Document 1, the applicants of the present invention have proposed a developing roller in which the first electrode and the second electrode are each formed by helically winding a metal foil having a width of approximately 30 μm around the outer circumferential surface of a cylindrical member (for example, see Japanese Patent Application Publication No. 2004-191835 (called Patent Document 2 below) and Japanese Patent Application Publication No. 2007-86091).
In the above developing roller described in Patent Document 2, however, the metal foils are wound helically, and hence gaps tend to be formed between the wound metal foils and the cylindrical member. The formation of gaps makes it difficult to manufacture the electrodes with desired accuracy (in other words, makes it difficult to form the electrodes exactly in desired positions).
Further, as described above, the gaps are formed between the wound metal foils and the cylindrical member because the electrodes are formed by winging the metal foils helically. With aging degradation, the gaps allow the metal foils, that is, the electrodes to get out of position on the cylindrical member or cause a problem in durability of the electrodes.